1. The Field of the Invention
The present invention relates to screening vertical cavity surface emitting lasers (VCSELs) for quality control purposes. More specifically, the invention relates to methods and apparatus for testing VCSELs that have been installed in transceiver modules in which damage to the laser may have taken place either during the installation into the transceiver module or some time after installation into the transceiver module.
2. Description of the Related Art
In the field of data transmission, one method of efficiently transporting data is through the use of optical fibers. Digital data is propagated through a fiber optic cable using light emitting diodes or lasers. Light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electromagnetic interferences that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber optic cable as can occur with electrical signals in wire-based systems. While there may be an evanescent field that enables one to siphon some portion of the light off the fiber by bending the fiber such that it is possible to tap fiber communications without breaking the fiber, it is in general much more difficult than for electrical communications. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire.
To accomplish communication in a fiber optic network, one component that is used is a transceiver module. A transceiver module has an optical input port and an optical output port. The optical input port is typically connected to a photodiode. The photodiode is connected to control circuitry within the transceiver module such that the combination of the photodiode and control circuitry can be used to monitor optical data received from the fiber optic network.
The optical output port typically is connected to a laser or a laser diode. The laser is also connected to the control circuitry. By modulating the signals to the light emitting diode or laser diode, digital optical data can be propagated from the transceiver module onto the fiber-optic network. The transceiver module also typically includes components for converting optical signals to electrical signals and electrical signals to optical signals so that electrical components in the network can communicate with the optical portions of the network.
In an 850 nanometer fiber-optic transceiver module, a particular laser known as a vertical cavity surface emitting laser (VCSEL) is often used. The emitting area of a VCSEL is defined by where the current flows through the quantum well region. Since the VCSEL is created as a uniform planar structure there is nothing initially to determine where the current will go. The two most common methods of solving this problem are: 1) disrupting the lattice structure by particle implantation (usually hydrogen, i.e., protons), which causes the current to preferentially flow through the non-disrupted region defined by the negative image of the implantation mask and 2) blocking the current by creating a dielectric oxide layer in the region surrounding the emitting cavity, commonly known as the oxide confinement technique. In recent times, the technique of oxide confinement has been used to create a variety of oxide laser diodes known as oxide defined vertical cavity surface emitting lasers more commonly known as oxide VCSELs. While the oxide lasers diodes exhibit some desirable characteristics, they also have the unfortunate drawback of being very susceptible to electrostatic discharge (ESD) damage. This is easy to predict since the lattice disruption confinement technique disrupts the layers all the way through the VCSEL structure, while the oxide confinement technique only creates a very thin dielectric layer. For any given applied voltage, the electric field strength will be proportional to the thickness of the dielectric region, so the thin oxide layer will have a much higher field across it and sustain damage at a lower voltage than the thick implanted layers. Whereas most typical electronics have an ESD resistance of least 500 V and often much higher, the oxide VCSEL is susceptible to ESD damage at voltages as low as 200 V or less. Another difficulty with ESD damage to oxide VCSELs is that such damage is often latent, meaning that it is not immediately detectable simply by observing the initial performance of the oxide VCSEL. However, ESD damage causes the oxide VCSEL to quickly degrade.
One method of screening oxide VCSELs to verify that they have not been damaged by an ESD is by examining the current/voltage characteristics of the oxide VCSEL. These characteristics are often shown in an I-V graph. By comparing the current/voltage characteristics of oxide VCSELs that are known to be operable and undamaged with the oxide VCSEL under test, one may be able to make a determination as to whether the oxide VCSEL under test has been damaged by ESD. Often, however, because the change in the current/voltage characteristics may be slight when ESD damage has taken place on an oxide VCSEL, the testing of the oxide VCSEL may need to be performed over time to ensure that there is no change in the current/voltage characteristics of the particular oxide VCSEL. In other words, it may not be sufficient to compare the current/voltage characteristics of the oxide VCSEL under test with known current/voltage characteristics, because oxide VCSELs have inherent variation in their current/voltage characteristics. Electrostatic discharge damage to an oxide VCSEL may appear to be simply a device variation.
While presently it is possible to use such testing to evaluate the condition of the oxide VCSEL prior to the oxide VCSEL being installed in another component such as a transceiver module, once the oxide VCSEL has been installed in another component testing can be much more difficult. Further exacerbating the problems with testing oxide VCSELs is the fact that installation of the oxide VCSEL into other components may actually be the cause of the ESD that damages the oxide VCSEL. It would therefore be beneficial if testing of the oxide VCSEL could be performed after the oxide VCSEL has been installed in another component or after the other component has been in service for a period of time.